1. Field of Invention
The present invention relates generally to hydrofoil structures commonly employed to generate lift or thrust for marine applications such as for example, foils of high speed hydrofoil crafts, blades of marine propellers, and impellers of fluid pumps or turbines, where a lift force is produced by movement of the hydrofoil structure relative to the surrounding water. More particularly, the present invention relates to hydrofoil structures for efficient operation over a wide speed range from subcavitating to supercavitating operation.
2. Brief Description of Related Art
There is presently a great interest in providing high speed ships and ship propulsors capable of efficient operation at high speeds. Hydrofoil crafts have been used where operation above 45 knots is desired. The hydrodynamic characteristics of lift producing hydrofoil structures are very similar to the subsonic aerodynamic characteristics of aircraft wings. Thus, it has been possible to adapt many airfoil theories and computational techniques to hydrofoil and propeller blade section designs. However, there exists a major distinction between hydrofoil structures and aircraft wings. Operated below the free surface, a hydrofoil or propeller will develop vortex cavitation and surface cavitation on the foil or blade above a certain critical speed. Cavitation inception occurs when the local pressure falls to or below the vapor pressure of the surrounding fluid. Cavitation inception can be predicted from the pressure distribution over the hydrofoil structures since the cavitation inception index .sigma..sub.i is equal to the negative minimum pressure coefficient -C.sub.pmin. Above the critical cavitation inception speed, serious fundamental flow changes occur that lead to undesirable variations in hydrodynamic characteristics (e.g., loss of lift in hydrofoils and thrust breakdown in propellers) and possible damage to foil or blade structure. Consequently, the major obstacle to achieving high sustained speeds in water is the occurrence of cavitation with its many detrimental effects. Consequently, the design philosophy for hydrofoil and propeller blade sections has been governed by the following requirements: (1) provide the required lift/thrust at a specified design point while ensuring adequate structural strength (especially at thin leading and trailing edges) for all operating conditions; and (2) avoid or minimize cavitation or the detrimental effects of cavitation. To this end, three distinct hydrofoil structures, i.e., subcavitating, base ventilated and supercavitating designs, have been proposed for use at different design speeds.
Subcavitating hydrofoil structures generally have conventional airfoil shaped profiles (cross-sectional shapes) selected, for example, from the NASA design literature, such as NACA 16-series or NACA 6-series laminar airfoils, and are designed to operate fully wetted over both the upper and lower surfaces. Such profiles derive most of their lift from their upper surfaces. However, the shape of the upper and lower surfaces influence the pressure distribution produced by the other. Thus, to produce an efficient hydrofoil structure having a high lift-to-drag ratio, the upper and lower surface shapes should be designed together as an integral unit. Subcavitating hydrofoil structures operate efficiently, with high lift-to-drag ratios, at speed up to the critical speed at which the hydrofoil begins to experience cavitation, i.e., the critical cavitation inception speed. The critical cavitation inception speed may be increased through design methods such as varying the profile geometric characteristics, e.g., lowering the camber (to reduce hydrodynamic loading at the expense of efficiency) and/or reducing the section thickness (to reduce suction pressure -C.sub.pmin at the expense of structural strength), or by restricting operation to lower sea states in order to reduce craft motions and maintain an angle of attack near the design angle. Typically, a subcavitating hydrofoil is efficient up to a critical speed of about 45 knots while a subcavitating propeller is efficient up to a critical speed of about 25 to 30 knots.
Due to the occurrence of cavitation, subcavitating hydrofoil structures are not practical for marine applications beyond the critical cavitation inception speed. To overcome the problems associated with cavitation on subcavitating hydrofoil structures, supercavitating hydrofoils and fully wetted base ventilated hydrofoils were developed in the 1960's for high speed marine applications.
Supercavitating hydrofoil structures are predominantly used at high speeds where subcavitating hydrofoil structures are impractical due to cavitation. Supercavitating hydrofoil structures generally have a triangular or wedge shaped profile with a sharp leading edge and a blunt trailing edge. Profile thickness typically increases from a minimum at the sharp leading edge to a maximum at the blunt trailing edge. The supercavitating condition is initiated at high speeds (supercavitating speeds) when the sharp leading edge causes formation of a fully developed cavity over the entire upper surface. Cavity collapse occurs well abaft of the trailing edge, thus, problems of buffeting and erosion associated with cavitation on subcavitating hydrofoil structures are avoided. The lift producing lower surface of a supercavitating profile is generally flat or concave and is designed using well known supercavitating theory to produce operating pressures greater than ambient pressure so as to prevent cavitation. It is noted that NACA sections, which have been extensively used in subcavitating hydrofoil and marine propeller design, typically have convex lower surfaces that are not efficient lift producers under supercavitating conditions. Because supercavitating profiles derive their lift primarily from increased pressure over the lower surface, with the upper surface exerting no influence on lift production at supercavitating speeds, the lower surface shape is designed with little or no regards to the upper surface shape. The shape of the upper surface is immaterial as long as it does not contact the cavity wall, i.e., the free-surface between the air or vapor filled cavity and the water. Therefore, the upper surface is generally flat, although it may have a slight curvature in order to provide thickness for strength.
To achieve a supercavitating condition, a supercavitating hydrofoil or propeller must operate at high speeds and low cavitation numbers. At supercavitating speeds, the presence of the cavity generates a cavity drag that lowers efficiency. Moreover, due to extreme inefficiency prior to achieving supercavitating conditions, supercavitating hydrofoil structures are impractical for low speed operation, thus, necessitating secondary means of producing lift or thrust at low speeds. To maintain a reasonable efficiency, the lower limit for application of supercavitating hydrofoils is approximately 50 knots while the lower limit for application of supercavitating propellers is approximately 45 to 50 knots. Below these speeds, only a partial cavity develops resulting in cavity collapse forward of the trailing edge causing buffeting and erosion. Additional obstacles associated with use of supercavitating hydrofoils include: the high angle of attack needed to generate a reliable, steady cavity compared to subcavitating hydrofoils, results in large drag and low efficiency, especially at off design speeds; due to increased form drag at low speeds the lift-to-drag ratio is small compared to subcavitating hydrofoils, consequently, supercavitating hydrofoils have difficulty generating sufficient lift for take-off while supercavitating propellers have difficulty generating sufficient thrust to overcome a ship's hump drag; and obtaining structural strength of the thin leading edge is difficult.
Base ventilated hydrofoil structures have been proposed for use at design speeds falling in the intermediate range between subcavitating and supercavitating speeds. Base ventilated hydrofoil structures are similar in shape to supercavitating hydrofoils in that they generally have triangular or wedge shaped profiles with blunt trailing edges. Base ventilated hydrofoil structures, however, have thicker or blunter leading edges than supercavitating profiles to prevent formation of a cavity over the entire upper surface. The profile thickness increases from leading edge to trailing edge so that base ventilated hydrofoils can operate cavitation free at higher speeds than subcavitating profiles at the expense of increased form drag and lowers efficiency. To partially compensate for the increased form drag, base ventilated hydrofoil structures have a gas introduced into the flow behind the blunt trailing edge resulting in lower form drag than supercavitating profiles. However, efficiency at low speeds is less than subcavitating hydrofoils and, because base ventilated hydrofoil structures are designed to operated with the upper and lower surfaces fully wetted, lift force produced is sensitive to variations in angle of attack.
Subcavitating hydrofoils for low speed operation (typically less than about 45 knots), supercavitating hydrofoils for high speed operation (typically above about 50 to 60 knots) and base ventilated for intermediate speed operation have been known for some time. FIG. 1 presents representative plots of efficiency (as represented by lift-to-drag ratio) as a function of speed for fully-wetted-subcavitating (curve A), base ventilated (curve B), and supercavitating (curve C) hydrofoils. However, presently, there is no hydrofoil or propeller design capable of operating over a wide speed range, i.e., a speed range that encompasses subcavitating, base ventilated and supercavitating operating ranges, without experiencing the problems described above. Consequently, hydrofoils have generally been limited to efficient operation in only one of the subcavitating, base ventilated or the supercavitating regimes. Therefore, there is a need to provide a hydrofoil structure for use as a hydrofoil or marine propeller that overcomes the problems and operational limitation associated with subcavitating, base ventilated and supercavitating hydrofoils and propellers.